The proposed research is concerned with problems of growth, morphogenesis and differentiation in the cellular slime molds, particularly Dictyostelium discoideum. The myxamoebae exist in the vegetative stage as independent amoeboid cells which subsequently aggregate to form colonies or pseudoplasmodia. The pseudoplasmodia undergo a series of morphogenetic movements accompanied by cell differentiation. These events result in the formation of a mature fruiting body composed of a slender stalk supporting a mass of spores. A major objective involves determining the ultrastructural components in the unit membranes responsible for maintaining cell adhesion upon which cell differentiation appears to depend. Unless aggregation and consequently cell association is accomplished differentiation cannot occur. Cell association is paralleled by major ultrastructural changes in the cell surface unit membranes. These changes observed by freeze- etch electronmicroscopy consist of a striking increase in the number and size of particles composing the unit membranes. The unit membranes may be studied by preparing an antiserum which specifically blocks cell contacts. By the use of ferritin-labeled antiserum the relationship between the unit membrane particles and the adhesive sites may be established. Also the effect of blocking antiserum on the differentiation of prespores and mature spores and stalks would be determined. The role of unit membrane components in cell adhesion and differentiation would be further studied using aggregateless mutants. The mutants which are known to have surface antigen defects would be examined to determine if their development failure might be attributed to unit membrane aberrations. In addition aggregate dissociating enzymes and 3',5'-cyclic AMP would be used to determine the nature of the particles and the factors controlling their synthesis. A variety of immunological and immunochemical methods combined with thin-section and freeze-etch electronmicroscopy techniques will be employed in the conduct of the research.